1. Field of the Invention
This invention relates to a fiber capable of rapidly shrinking when coming into contact with water at ordinary temperature and capable of maintaining the fiber form and exhibiting performance characteristics such as rubber elasticity, to a yarn made by using said fiber, capable of rapidly shrinking in the lengthwise direction upon absorption of water at a high rate of appearance of shrinkage stress and capable of exhibiting high strength and rubber elasticity for a prolonged period of time even after water absorption and shrinkage, and to a water-absorbing shrinkable material made by incorporating into a nonwoven fabric shrinkable upon absorption of water a yarn (yarn, twist yarn, cord, or the like) made from said fiber and shrinkable more rapidly and to a greater extent as compared with said nonwoven fabric.
2. Description of the Prior Art
Fibrous materials which, when immersed in water, can absorb water under swelling or dissolution have been known so far. For instance, polyvinyl alcohol (hereinafter abbreviated as "PVA")-based water-soluble fibers derived from hydrophilic group-containing resins, and highly water-absorbent fibers modified by incorporation of hydrophilic groups while they are in fibrous form are known widely. However, these fibers are used either as fibrous adhesive for other materials or for the purpose of temporarily fixing other raw materials to maintain their processability. In the latter case, said fibers are finally removed by dissolution. In some instances, such fibers are used for the mere purpose of water absorption. Thermally shrikable fibers made of hydrophobic resins are also known but they are shrinkable only in a relatively high temperature range exceeding 60.degree. C.; they solidify upon cooling and therefore do not show rubber elasticity.
The present inventors have previously proposed a fiber made of PVA and capable of shrinking in water to a great extent in Japanese Laid-Open Patent Publication No. 60-2709. However, this fiber required 30 seconds for 30% shrinkage even in water at a temperature of 30.degree. C. and shows a very slow rate of shrinkage when ordinary temperature water (in the neighborhood of 20.degree. C.) is used, although it is excellent in that it is hardly soluble.
A primary object of the invention is to provide a fiber which can respond to water very sensitively when in comes into contact with water at 20.degree. C., for instance, and can shrink greatly without substantial dissolution.
Yarns capable of shrinking upon absorption of water and knitted or woven fabrics made of such yarns are useful in a variety of applications. For example, when such shrinkable yarns are used in lengthwise edge portions of disposable diapers, the yarns absorb water excreted by the users of the disposable diapers (mainly water contained in urine, diarrheal stool, etc.) and shrink, so that urine, stool and the like can be prevented from escaping from such disposable diapers (e.g. U.S. Pat. No. 4,357,938 and West German Patent No. 3,130,241). When said shrinkable yarns are used as various fastening strings and allowed to absorb water in the fastened portions, they can prevent the fastened portions from loosening thanks to the appearance of shrinkage stress upon absorption of water. When cloths made of such shrinkable yarns are used as water roll coverings in offset printers, cylinders somewhat greater in diameter than the core roll are made of said cloths in advance. When they are mounted on the core roller and immersed in water, the cylindrical coverings can come into close contact with the core roll, so that the mounting procedure can be much simplified. Furthermore, coarse-textured cylinders or cords made of said shrinkable yarns can be mounted on plant roots on the occasion of plant transplantation in a simple manner for the same reason as in the case of the above-mentioned water roll coverings for offset printers. In addition, such yarns are useful in manufacturing nets or strings for ham and the like. Cloths of said kind as made for civil engineering purposes, when once subjected to water absorption treatment, for instance, acquire very high shock resistance with high-degree elongation and can be used as excellent sheets for preventing bank-protecting basements from being washed away or scooped out by waves, for instance. The yarns are also useful as humidity sensors.
As is suggested by the applications mentioned above, it is essential that water-absorbing shrinkable yarns should meet four requirements, namely high rate of shrinkage and high of appearance of shrinkage stress upon absorption of water, retention of high strength for a prolonged period of time even after absorption of water, and appropriate degree of rubber-like elasticity. These requirements should be met at the same time.
Various yarns capable of shrinking upon absorption of water have been so far proposed. However, the prior art water-absorbing shrinkable yarns cannot meet at least one of the above four requirements, hence are never satisfactory. For instance, those yarns that show increased rate of shrinkage and increased rate of appearance of shrinkage stress upon absorption of water contain a hydrophilic polymer as a fiber-constituting polymer and have a structure such that water can easily invade the fiber inside. Furthermore, such yarns have an increased degree of orientation of fiber-constituting molecules so that fibers swelled with invading water can contribute to shrinkage in the direction of fiber axis. In this manner, such fibers have a structure such that they can swell freely upon absorption of water. Therefore, the fiber inside structure can be readily disintegrated upon absorption of water and, as a result, the strength and shrinkage stress after absorption of water and shrinkage decrease rapidly. A conceivable method of increasing the strength and shrinkage stress after water absorption and shrinkage would comprise twisting said yarns to a high number of twist so that the internal structure can hardly be disintegrated. Since, however, such yarns have a compact structure, water can hardly invade the yarn inside and at the same time the free space in which fibers can swell is limited even when yarn-constituting fibers absorb water. For these and other reasons, the rate of shrinkage and rate of appearance of shrinkage stress upon absorption of water are markedly reduced. Thus, the hitherto known water-absorbing shrinkable yarns are deficient in at least one of the performance characteristics, rate of shrinkage upon absorption of water, rate of appearance of shrinkage stress, strength after water absorption and shrinkage and rubber-like elasticity after water absorption and shrinkage, and accordingly encounter problems in their use in various fields such as mentioned above.
Accordingly, it is a second object of the invention to provide a water-absorbing shrinkable yarn which can meet all the above four requirements, namely a yarn which is high in rate of shrinkage upon absorption of water, high in rate of appearance of shrinkage stress and high in strength after water absorption and shrinkage, and has rubber-like elasticity after water absorption and shrinkage.
The prior art relative to the above-mentioned disposable diapers is described in the following in further detail. While various disposable diapers have been proposed so far, the edge portions of disposable diapers should have a function such that said edge portions should be in close contact with the thigh in order to prevent leakage of urine and so forth. The following three structures are known as the means therefor.
(a) The use of an elastomer (in the form of a tape or yarn), such as polyurethane or rubber, in edge portions of disposable diapers. Said edge portions are kept in close contact with the thigh of users by the elasticity of said elastomer.
(b) The use of yarns shrinking upon absorption of water in edge portions of disposable diapers. Said water-absorbing shrinkable yarns absorb urinary water and so on and said edge portions are brought into contact with the thigh of users through the shrinkage taking place on that occasion.
(c) Combined use of the above methods (a) and (b).
Among these three methods, method (b) is preferred to method (a) because packed products before use are less bulky and more handy to carry and furthermore less causative of stuffiness. Therefore, they have been commercialized recently. An example of the transverse section of such a product is shown in FIG. 10, where 15 is a back sheet made of a polyethylene film about 25 .mu.m in thickness, 16 is a water absorbent for absorbing water such as urine, 17 is a front sheet, 18 is a water-absorbing shrinkable yarn for bringing the edge portion of the disposable diaper into close contact with the thigh, and portions B are sealing portions for securing water-absorbing shrinkable yarns at predetermined sites (the back sheet and front sheet are adhered together in said portions). Disposable diapers which use this method (b) indeed have outstanding features such as mentioned above but are still disadvantageous in that when a larger quantity of water than that absorbable by the water-absorbing shrinkable yarn reaches an edge portion, the shrinking of the water-absorbing shrinkable yarn is too late to prevent water from escaping out of the edge portion. The prior art avoids this phenomenon by positively introducing part of water absorbed in the water absorbent to the water-absorbing shrinkable yarn by utilizing an auxiliary means such as an absorbent paper as early as possible so that the shrinkage of the water-absorbing shrinkable yarn can be caused prior to spontaneous arrival of water absorbed by the water absorbent at the edge portion (the absorbent paper shown by 19 in FIG. 10 functions in this manner), as disclosed in Japanese Laid-Open Patent Publication No. 57-35002 (i.e. the above-cited U.S. Pat. No. 4,357,938 or the above-cited West German Patent No. 3,130,241) or Japanese Laid-Open Patent Publication No. 57-56502, for instance. However, this technique, too,,has failed to solve the leakage problem since the shrinkage of the water-absorbing shrinkable yarn cannot take place in time when diapers are used in an extraordinary condition (e.g. when users urinate while lying on their side or when users discharge diarrheal stool violently).
When, in using a water-absorbing shrinkable yarn in edge portions of disposable diapers, the water absorbent and the water-absorbing shrinkable yarn are superposed on each other, the edge portions fail to shrink because the water absorbent itself is bulky and does not shrink. Therefore, it is essential in the prior art that the water-absorbing shrinkable yarn should be located almost alone in the edge portions of disposable diapers without overlapping with the water absorbent (cf. the above cited Japanese Laid-Open Patent Publication No. 57-35002 and No. 57-56502). The water-absorbing shrinkable yarns to be used in the edge portions of disposable diapers generally have a diameter of about 0.5-0.7 mm and therefore its single use may bring about an occurrence of impressions caused by constriction on the thigh when said portions are kept in close contact with said thigh. Although such impressions are harmless to users from the medical viewpoint, they may serve as a factor making mothers anxious about them especially when the users are their babies. This is also a great problem which method (b) faces.
Method (c) has the same problem as mentioned above since it is a mere combination of method (a) and method (b).
Accordingly, it is a third object of the invention to provide a water-absorbing shrinkable material usable in manufacturing disposable diapers having ideal performance characteristics, namely such that when used as a shrinkable material in edge portions of disposable diapers, said water-absorbing shrinkable material prevents leakage from said disposable diapers without causing impressions of constriction on the thigh with little stuffiness in the crotch and also renders compact the folded form of the disposable diapers before use.